1. Field of the Invention
The present invention relates to a circuit, for example, for driving the film winder by means of a motor when the film in a camera is wound or rewound, particularly a motor drive circuit for a small device with a built-in battery, such as for a camera, in which the utilization efficiency of the battery is improved when the motor is driven.
2. Description of the Prior Art
Quite recently in the field of the camera, film winding shutter charging and lens focusing have become more automated. For example, in the case of film winding in general, the motor drive circuit for automatically winding the film is operatively engaged with the release of the camera in order to start the film winding motor immediately after the release, whereby the film for one picture frame is mechanically wound by the drive of the motor, and the number of the film perforations is detected in order to stop the motor. FIG. 5 shows an embodiment of the conventional motor drive circuit.
In FIG. 5, position 1 of the change over switch SW is the position taken when the film has been wound, position 2 is that taken when the film is to be wound and position 3 is that taken when the film is to be rewound. When the winding is complete, condenser C and motor M are short-circuited by the contact of switch SW at the position 1. When the change over switch SW is changed over to the position 2, in order to wind the film, the power source is switched on in such a manner that the base current flows through transistor Q2 via resistor R3 in order to bring transistor Q2 to the on state. The base current is supplied to transistor Q3 via resistor R4 to bring transistor Q2 to the on state, whereby motor M starts to wind the film.
When the film for one picture frame has been wound, the perforation of the film for the picture frame is detected and switch SW is mechanically moved to position 1.
When the film comes to the end and further winding is not possible, the lapse of a certain time, set by the value of the capacitance C and the resistor R, which is longer than that necessary for the film winding, allows the terminal voltage of the condenser C to reach a value sufficient to supply the base current to transistor Q1 which brings transistor Q1 to the on state so that transistors Q2 and Q3 are brought to the off state in order to stop the current supply to the motor. In the case of rewinding, switch SW is changed to position 3 and current is supplied to the motor independently of the motor driving time, set by the condenser C and the resistor R, in such a manner that the film is continuously rewound.
In the above circuit construction, the base current I.sub.B of transistor Q3 during the motor operation is as follows: ##EQU1## Hereby: V.sub.BAT : Power source voltage
V.sub.CE : Switch on voltage between collector and emitter of Q2 PA1 V.sub.BE : Voltage between base and emitter of Q.sub.3
wherein the dependency of V.sub.CE and B.sub.BE on the power source voltage is comparatively little while I.sub.B largely depends upon V.sub.BAT and R4.
On the other hand, in order to drive the motor in the camera, it is sufficient to supply a current corresponding to the load of the motor which may be film advancing or the charging of the various circuits. Generally, it is necessary that the motor be driven even when the power source voltage is low, for example, due to the consumption, in order to extend the life of the battery. In case the value of the R4 is made smaller to the extent that a necessary current can be delivered to transistor Q3 even when the power source voltage is low, the base current flowing through R4 is large, as is obvious from the above relation, when the power source voltage is high enough to drive the motor.
However, the increased part of the base current does not directly contribute to driving the motor. Further, the higher the power source voltage is, the less negligible the increase part of the base current is, making it necessary to avoid this wave of power consumption.
On the other hand, it is well known that the lower the current consumption of the battery, the larger the energy obtainable from the battery and the better the utilization efficiency of the battery.